1. Field of Invention
This invention relates to telecommunications systems and related hardware. Specifically, the present invention relates to filters for Code Division Multiple Access (CDMA) wireless communications systems employing a plurality of carrier signals.
2. Description of the Related Art
Wireless communications systems are employed in a variety of demanding applications including urban mobile telephone and business applications. Such applications require reliable communications systems that can efficiently accommodate increased demand while minimizing dropped calls.
A wireless communications system, such as a Code Division Multiple Access system (CDMA), typically includes a plurality of mobile stations (e.g. cellular telephones or wireless phones) in communication with one or more base stations or Base Station Transceiver Subsystems (BTS), also called cell sites. The communications link from a BTS to a mobile station is known as the forward link. The communications link from the mobile station to the BTS is known as the reverse link.
A base station or BTS facilitates call routing among mobile stations and between mobile stations and other communications devices that are connected to the Public Switched Telephone Network (PSTN), also called the landline network.
CDMA communications systems are often built in accordance with the IS-95 telecommunications industry standard. In IS-95 systems, data is transmitted between a BTS and a mobile station in 20 millisecond frames. The frames are encoded digitally for channel noise, capacity, and data security reasons. A convolutional encoder often facilitates the encoding of the information within each frame.
To increase the reliability and capacity of CDMA communications systems, newer CDMA systems, called 3xc3x97CDMA systems are employed. 3xc3x97 CDMA systems are either 3xc3x97Direct-Spread (3xc3x97DS) or 3xc3x97Multi-Carrier (3xc3x97MC) systems. A 3xc3x97DS system is similar to an IS-95 system but transmits at three times the IS-95 chip rate (3xc3x971.2288 Mchips/s). A 3xc3x97MC signal is a 3xc3x97 bandwidth signal having three carrier signals with center frequencies spaced 1.25 MHz apart. The 3xc3x97MC signal includes left, center, and right carriers.
Before transmission in a 3xc3x97CDMA system (CDMA2000 system), communications signals are encoded, interleaved, scrambled, and multiplexed onto three data streams. Each data stream is transmitted via one of three different carrier signals (carriers) yielding a 3xc3x97 bandwidth signal. In a wireless phone, the 3xc3x97 bandwidth signal is downconverted to baseband, filtered, and then sent to an associated Mobile Station Modem (MSM). To demodulate the received 3xc3x97MC signal, the MSM requires that the 3xc3x97MC filter be lowpass filtered. The lowpass filter must maximize the Signal-to-Noise Ratio (SNR) by extracting each carrier signal from the received 3xc3x97 bandwidth signal with small inter-carrier interference and by closely matching base station pulse shaping effects. The filter should also accommodate the effects of base station pre-equalization on the received signal. Unfortunately, before the present invention, a lowpass filter that sufficiently performs these functions was not available.
Existing low-pass filters neither effectively compensate for base station pre-equalization filtering, nor extract each carrier signal from the 3xc3x97 bandwidth filter, nor closely match the base station""s pulse shaping. Consequently, existing filters may result in undesirably large inter-carrier interference and may fail to minimize overall phase nonlinearly of the received signal, which degrades signal quality.
Hence, a need exists in the art for an efficient filter for demodulating a received 3xc3x97 bandwidth CDMA signal that minimizes inter-carrier interference, compensates for base station pre-equalization filtering, and that closely matches the base station""s pulse shaping.
The need in the art is addressed by the multi-carrier filter for a wireless communications system employing a multi-carrier signal of the present invention. In the illustrative embodiment, the inventive filter is adapted for use with a multi-carrier wireless CDMA system and includes a first mechanism for receiving the multi-carrier signal and extracting constituent carrier signal components in response thereto. A second mechanism filters the carrier signal components and outputs a demodulated and filtered multi-bandwidth signal in response thereto.
In a more specific embodiment, the first mechanism includes a rotator. The multi-carrier signal is a 3xc3x97 bandwidth multi-carrier signal having three carrier components. The three carrier components include a center carrier, a left carrier, and a right carrier. The center carrier, the left carrier, and the right carrier are separated by approximately 1.25 MHz. The rotator is a lookup table rotator that includes a mechanism for selectively rotating the multi-carrier-signal clockwise or counter clockwise and outputting the left carrier or the right carrier, respectively, in response thereto.
In the illustrative embodiment, the second mechanism includes an Infinite Impulse Response (IIR) filter for matching base station pulse shaping associated with the multi-carrier signal. The second mechanism further includes a mechanism for compensating for pre-equalization effects on the multi-carrier signal to minimize phase non-linearity. The IIR filter is a 5th order elliptic IIR filter, which is employed as a 1xc3x97 lowpass filter for lowpass filtering the individual carrier signal components. The carrier signal components include three data streams separated in frequency.
The IIR filter has a cascade direct form II filter structure and is characterized by the following transfer function:       H    ⁢          (      z      )        =            α      00        ⁢                            (                                    β              00                        +                                          β                01                            ⁢                              z                                  -                  1                                                      +                                          β                02                            ⁢                              z                                  -                  2                                                              )                          (                      1            +                                          α                01                            ⁢                              z                                  -                  1                                                      +                                          α                02                            ⁢                              z                                  -                  2                                                              )                    ·                        (                                    β              10                        +                                          β                11                            ⁢                              z                                  -                  1                                                              )                          (                      1            +                                          α                11                            ⁢                              z                                  -                  1                                                              )                    ·                        (                                    β              20                        +                                          β                21                            ⁢                              z                                  -                  1                                                      +                                          β                22                            ⁢                              z                                  -                  2                                                              )                          (                      1            +                                          α                21                            ⁢                              z                                  -                  1                                                      +                                          α                22                            ⁢                              z                                  -                  2                                                              )                    
where xcex100, xcex101, xcex102, xcex111, xcex121, xcex122, xcex200, xcex201, xcex202, xcex210, xcex211, xcex220, xcex221, and xcex222, are predetermined constants, and z is a complex variable.
In the specific embodiment, xcex100≈22/512, xcex101≈xe2x88x92895/512, xcex102≈414/512, xcex111≈xe2x88x92445/512, xcex121≈xe2x88x92921/512, xcex122≈476/512, xcex200≈7/64, xcex201≈xe2x88x923/64, xcex202≈7/64, xcex210≈4/64, xcex211≈4/64, xcex220≈88/64, xcex221≈xe2x88x92104/64, and xcex222≈88/64. The values of the xcex1 and xcex2 coefficients are selected to avoid overflow at nodes of the IIR filter. The xcex1 and xcex2 coefficients are implemented via shifting-and-adding-type multipliers, which scale signal values at the nodes of the IIR filter in accordance with the L-infinity norm.
The novel design of the present invention is facilitated by the second mechanism, which includes unique IIR filters that help minimize inter-carrier interference, compensate for base station pre-equalization filtering and pulse shaping, and achieves an excellent Signal-to-Noise (SNR) ratio.